Offset lithography is the one of the most widely used forms of printing and is characterized by having the image and non-image areas in the same plane (planographic). The image and non-image areas have different wetting characteristics such that, on press, the hydrophilic (water-loving) non-image areas of the printing plate are wetted by water or a dampening solution and only the hydrophobic or oleophilic (oil-loving) image areas are wetted by ink, which is subsequently transferred or ‘offset’ onto the surface of a material upon which the image is to be reproduced, such as paper, cloth or plastics, commonly by the use of an intermediate ‘blanket’ roller.
Many long-run, commercial, planographic printing plates comprise a grained and anodized aluminium layer on which is coated an ink-receptive, hydrophobic layer. Initiating the production of image and non-image areas is usually achieved by selectively exposing the plate to some form of radiation, for example ultra-violet, visible or infra-red radiation. The plate is then processed to remove unwanted, non-image areas of the ink-receptive layer and reveal the appropriate non-image areas of the hydrophilic layer beneath. This can be done in a number of ways, for example, by using a chemical process, by dissolution, by washing, sometimes with some abrasion, or by ablation, all using specific stand-alone devices or performed ‘on-press’. It is often necessary to apply a layer of gum to the non-image areas to improve the plate performance. Waterless planographic printing plates use an overlying silicone layer to form the non-image areas, since silicone has a very low surface energy and is not wetted by the special lithographic ink used for waterless printing. In common with the more conventional printing plates, a processing step removes the unwanted material, in this case the selected areas of the silicone layer, to reveal appropriate ink-receptive image areas beneath.
The wetting characteristics of solid surfaces are governed by the chemical properties and the microstructure of the surfaces. It is well-known that both hydrophilic and hydrophobic surface properties can be enhanced by certain levels of surface roughness (see for example, “Physical Chemistry of Surfaces”, A. W. Adamson, 5th edition, Wiley & Sons, New York, 1990) and many plates, such as for example those comprising grained and anodized aluminium, rely on such an effect to produce hydrophilic surfaces suitable as non-image areas on lithographic printing plates.
The production of the image and non-image regions prior to printing by the exposure of a plate selectively to some form of radiation can in some cases be based on the use of metal oxides, sulfides or nitrides coated as thin layers onto a suitable substrate. In particular, it is known that some metal oxides, when exposed to ultra-violet light with sufficient energy to promote an electron into the conduction band, become substantially more hydrophilic and hence are ‘radiation-sensitive’. In addition, the same exposure process can also result in the destruction of organic materials that are in contact with the metal oxide surface (see for example, “Photocatalytic Purification of Water and Air”, D. F. Ollis and H. Al-Ekabi, Proceedings of the First International Conference on TiO2 Photocatalytic Purification and Treatment of Water and Air, London, Ontario, Canada, 1993, Elsevier, Amsterdam). If the organic material is hydrophobic, the resulting difference in wetting characteristics between exposed and unexposed areas may be sufficient to allow lithographic printing to take place.
Examples of the use of such photocatalytic mechanisms for printing plates are described by Suda et al. in a number of patents and applications, including US Patent Publication No. 2005/0092198 and U.S. Pat. Nos. 6,732,654; 6,978,715; 6,637,336; 6,564,713 and 7,205,094. In such cases the photocatalyst layer contains titanium dioxide, preferably with significant anatase form, and the organic hydrophobic material is coated as a separate, overlying layer. These patents disclose different means of applying the particulate photocatalyst and the hydrophobic layer, including the use of Chemical Vapour Deposition, which is a process wherein the photocatalyst is formed in a vapour phase above the substrate and then adheres to the substrate, often as multiple layers. US Patent Publication No. 2005/0092198 discloses that this process should be carried out from 400° C. to 800° C.
Kobayashi et al. describe in US Patent Publication No. 2008/0085478 that a similar titanium dioxide layer structure can be used for a number of applications, including printing. Kobayashi et al. also describe in U.S. Pat. No. 7,252,923 a ‘two-part’ process, whereby the titanium dioxide photocatalyst layer and organic hydrophobic material are separate elements which are brought into contact temporarily such that, upon selective exposure to ultra-violet radiation, the photocatalyst layer changes the wetting character of the other element. After exposure the two sheets are separated, leaving a printing plate comprising hydrophilic and hydrophobic regions and a photocatalyst element that can be reused.
Kasai et al. in U.S. Pat. No. 6,232,034 discuss the use of anatase titanium oxide particles in a matrix of a hydrophobic material that changes wetting characteristics on exposure to light. Similarly, Akio et al. in JP 11305422 describe photocatalyst particles which become hydrophilic on exposure to light. Ooishi et al. in U.S. Pat. No. 6,833,225 disclose making a printing plate by sputtering titanium dioxide onto pre-grained aluminium and then treating this surface with a hydrophobic material.
Riepenhoff et al. in U.S. Pat. No. 6,976,426 describe a process of making a reusable printing form comprising a particulate photocatalyst layer combined with a binder that changes wetting characteristics when exposed to either ultra-violet light or heating. Ikeda et al. in JP 10250027 disclose a printing system that uses a titanium dioxide plate which can be erased by heat, allowing for its reuse. The titanium dioxide used is particulate and forms the major part of the coating, the remaining materials in the coating being unspecified. Mori et al. in U.S. Pat. No. 7,032,514, describe a planographic printing press comprising a printing plate with a photocatalyst layer formed on a heat insulating layer and a means of applying a hydrophobic layer, whereby the wetting characteristics are changed when exposed to either ultra-violet light or an additional heat treatment in the range 40° C. to 200° C.
A different approach from utilizing vapour-deposited metal inorganic layers to make printing plates is disclosed by Ellis et al. in U.S. Pat. No. 6,045,964. In this method the printing member comprising a metal oxide layer is exposed in an imagewise fashion using an infrared laser so as to remove by ablation selected areas to form a printing plate.